Heres a small script of what i think....
Kairi walking on a hill in the sunset after school (you see it in a trailer)
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Sora, Donald and Goofy are walking out of the castel talkin (Sora is in new clothes). You get to practice walking around some sort of feild and get to choose and collect things. like in KH1.
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It switches to BHK and his freinds in twilight town. Sitting on top of a clock... after a while they get down and you can control him. you get to walk around twilight town and collect things and do stuff like of KH1.
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Sora desides to go back to hollow bastion but when he gets there he is confronted with thousands of heartsless. (u see in a trailer)
u are timed to see how many heartless u can kill in the time limit.
When Sora finally beats all the heartless Sora is confronted with a nobody.
the nobody castes som sort of spell on Sora and then dissaperas.
Sora collapses. and there is a sudden switch to Kairi.
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(Kairi is standing on the end of the beach like u see in pics)
Kairi gaspes and lookes out to sea (she can feel soras heart)
Selphie walks up to Kairi slowly and asks whats wrong.
Kairi is silent for a while and then speaks "His voice...its left me.."
Selphie is confused and askes.....blah blah..
i cant think of any more but thats how it think how sora turns...whateva.